1. Field of the Invention
The present invention relates to a Phase Locked Loop (PLL) circuit, a communication device using the PLL circuit, and a frequency adjustment method of a PLL circuit. More particularly, the present invention relates to a PLL circuit to be used in a portable communication apparatus, a communication device using the PLL circuit, and a frequency adjustment method of a PLL circuit.
2. Description of the Related Art
In portable communication apparatus such as a portable telephone, and so on, a PLL circuit is used as a local oscillator which is stable in frequency.
In FIG. 7, the block diagram of a known conventional PLL circuit is shown. In FIG. 7, a PLL circuit 1 is composed of a voltage-controlled oscillator 2, a TCXO (temperature compensation crystal oscillator) 3, frequency dividers 4 and 5, a phase comparator 6, a charging pump 7, and a loop filter 8. The voltage-controlled oscillator 2 is composed of an amplifier portion 2a, a resonance circuit portion 2b, a varactor diode VD in the resonance circuit portion 2b, and a trimming area 2c as an oscillation frequency adjustment mechanism. The trimming area 2c is formed of, for example, an electrode constituting a part of a resonance system. The resonance frequency of the resonance circuit portion 2b is changed by cutting and trimming the electrode and, accordingly, the oscillation frequency of the voltage-controlled oscillator 2 can be adjusted. The voltage-controlled oscillator 2 is provided with a control voltage terminal 2d for changing the oscillation frequency by applying a direct-current voltage to the varactor diode VD.
The output of the voltage-controlled oscillator 2 is divided and a part of the output is connected to the phase comparator 6 via the frequency divider 4. The TCXO 3 is connected to the phase comparator 6 via the frequency divider 5. The output of the phase comparator 6 is connected to the control voltage terminal 2d of the voltage-controlled oscillator 2 via the charging pump and loop filter in sequence. The frequency dividers 4 and 5, phase comparator 6, and charging pump 7 are not necessarily formed separately, typically being constructed in one PLL-IC.
Referring to the PLL circuit 1 of FIG. 7, because the oscillation frequency of the voltage-controlled oscillator 2 may not necessarily be attuned to an intended frequency in the initial state, the PLL circuit 1 is provided to adjust the oscillation frequency of the voltage-controlled oscillator 2. Adjustment of the oscillation frequency of the voltage-controlled oscillator 2 requires that a predetermined direct-current voltage be applied to the control voltage terminal 2d, and therefore the PLL circuit 1 in its entirety must be operated. That is, the oscillation frequency of the voltage-controlled oscillator must be adjusted while the PLL circuit is in operation.
The adjustment the oscillation frequency of the voltage-controlled oscillator 2 while the PLL circuit 1 is in operation requires a computer, or like equipment, for controlling the PLL circuit 1. The oscillation frequency adjustment also requires equipment for measuring the direct-current voltage applied to the control voltage terminal 2d, as well as a power source for operating the PLL circuit 1. Accordingly, the requirement for such measuring equipment gives rise to the drawback of increased equipment costs for the PLL circuit 1. Furthermore, it is necessary to perform the phase lock of the PLL circuit 1, which disadvantageously requires extra time for adjusting the oscillation frequency.